The present invention is related to nuclear magnetic resonance imaging, and more particularly to an NMR RF coil that reduces acoustic noise.
Magnetic Resonance Imaging (MRI) systems which use nuclear magnetic resonance to provide sectional images of a body are well known in the art. MRI systems basically comprise a main magnet for applying a static magnetic field; a plurality of gradient magnets, which are pulsed to select a layer or xe2x80x9cslicexe2x80x9d of a body to be imaged; and an RF coil which is applied to excite the atomic nuclei of hydrogen atoms in the body. In operation, a body to be examined is introduced into a strong, homogeneous, static magnetic field produced by the main magnet. This field causes alignment of nuclear spins of the atomic nuclei of hydrogen or other atoms in the body. After the nuclei are aligned, the RF coil is pulsed to excite precessional motion of the nuclei at a characteristic or Larmor frequency, and the gradient magnets are pulsed to select a layer or slice of the body for imaging. At the end of each radio frequency excitation pulse, the atomic nuclei initially precess and then settle back to a position determined by the applied static field. To generate an image, the RF signals from the nuclei are analyzed based on the spatial spin density or the distribution of the relaxation times in a layer or xe2x80x9cslicexe2x80x9d of the body as selected by the applied gradient fields.
MRI imaging has become a very important non-invasive medical analysis tool over the last twenty years. There are, however, a number of problems associated with the use of MRI in medical applications. The combination of high magnetic fields and gradient switching sequences, for example, results in induced magnetic forces that cause a significant amount of acoustic noise. The acoustic noise can cause discomfort to both the patient and medical personnel, and is therefore regulated by both FDA and OSHA regulations. Importantly, these regulations limit the pulse rates at which gradient magnets can be switched, thereby preventing important imaging techniques from being applied in medical imaging applications.
Although there are a number of sources of acoustic noise in an MRI system, one source has been traced to the RF coil. Typical RF coils are constructed as birdcage coils, such as the coil disclosed in U.S. Pat. No. 4,783,641. These birdcage coils comprise ladder-like resonant networks constructed of solid conductors. The conductors employed in these devices are generally selected to be relatively wide, in order to reduce the resistance in the coil. As the gradient coil is pulsed, a time varying magnetic field is applied to the conductors, and eddy currents develop in the conductive material. The eddy currents, in turn, induce a perpendicular Lorentz force in the coil which causes the coil to vibrate and produce acoustic noise on the coil cylinder. The wide conductors in the coil allow the circulating eddy currents to develop in relatively large loops, which therefore produce substantial Lorentz forces and acoustic noise in the system. While problematic in virtually any MRI system, these noise sources have become increasingly problematic as switching sequences have become more rapid.
There is a need, therefore, for an NMR RF birdcage coil for use in an MRI medical imaging system that produces a lower degree of acoustic noise as compared to prior art devices.
The present invention comprises a xe2x80x9cquietxe2x80x9d RF birdcage coil, the coil comprising a plurality of I-shaped elements, wherein each I-shaped element includes a leg and first and second end segments. Successive I-shaped elements are electrically coupled together with tuning capacitors which provide resonant signals at selected frequencies. In the RF birdcage coil of the present invention, a number of steps are taken to reduce eddy currents in the conductors of the coil, thereby lowering the acoustic noise produced by the coil as compared to prior art devices. A number of slotted or non-conductive channels can be provided in the conductive material of the RF coil to minimize the size of the conductive paths available for eddy currents. Furthermore, blocking capacitors can be applied between the legs and the end segments of the I-shaped elements to isolate the end segments from the legs at low frequency harmonics, such as those associated with gradient induced eddy currents, while acting substantially as a short circuit at high operational frequencies. At low frequencies, therefore, the conductive area in which eddy current loops can flow is again reduced, while operation remains substantially the same at high frequencies. Preferably, the blocking capacitors are constructed by providing the legs of each I-beam on a first side of a dielectric material, and the end segments on an opposing side. The blocking capacitor results from overlapping the conductive material forming the leg and end segment on opposite sides of the dielectric material, thereby minimizing the number of parts and increasing reliability of the coil.
These and other objects, advantages and aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made therefor, to the claims herein for interpreting the scope of the invention.